Navigating an enormous code base

[John Yates <john@yates-sheets.org>]

At my day job I work on a 35 year old product with literally
a few hundred million lines of codes. It is essentially a
mono-repo.

There are thousands of directory trees with Makefiles. To be
consistent with Emacs concepts, let's call these 'projects'.
Luckily projects are rarely nested, and not in the areas I work
on. There is no discipline requiring file names be unique across
projects, nor even within projects. Furthermore, within any
project there can be multiple unittest/ and/or pkgtest/
directories. Each such directory must contain a
suite_registration.cpp.

Most often I work within a single project and want to navigate to
files in the same project. Less frequently I need to find a file
in a sibling project. Unfortunately, there are ~400 siblings,
though I very rarely visit more than 10.

With recent attempts to work within Emacs' built-in tools, I
wonder how I find files in this code base. Or if Emacs lacks
adequate built-in tooling, what package(s) might help?